If you blame changes in the weather for your headaches, well, then you're absolutely right. According to a new study, high temperatures and low air pressure trigger migraines.

If you blame changes in the weather for your headaches, well, then you're absolutely right. According to a new study, high temperatures and low air pressure trigger migraines.

A study of more than 7,000 patients, led by researchers at Beth Israel Deaconess Medical Center (BIDMC), provides some of the first large-scale data on how environmental conditions.

According to the study, which has been published in journal Neurology, higher temperatures, and to a lesser degree, lower barometric pressure, contribute to severe headaches.

"Migraine headaches affect a large proportion of the population," notes Kenneth Mukamal, MD, MPH, the study''s first author and a physician in the Division of General Medicine and Primary Care at BIDMC.

Knowing that migraines can be set off by "triggers," including certain foods, alcohol, stress and hormones, Mukamal and his coauthors decided to study whether environmental factors were also acting as headache triggers.

"Air temperature, humidity and barometric pressure are among the most frequent reasons that people give for their headache pain," explains Mukamal, who is also an Associate Professor of Medicine at Harvard Medical School.

"But none of these reasons have been consistently verified. We wanted to find out if we could verify this ''clinical folklore.'' We also wanted to determine whether air pollutants trigger headaches, much as they have been found to trigger strokes," the expert added.

To reach the conclusion, Mukamal and his coauthors designed a "case crossover" study, which directly compares levels of pollutants and meterological variables at the time of the patient''s hospital visit with corresponding levels on preceding days and subsequent weeks. The study looked at 7,054 patients who went to the emergency room of Beth Israel Deaconess Medical Center between May 2000 and December 2007 and were discharged with a primary diagnosis of headache (2,250 diagnosed with migraine; 4,803 diagnosed with tension or unspecified headache).

Using meterological and pollutant monitors, they then compared measurements of a number of environmental factors - air temperature, barometric pressure, humidity, fine particulate matter, black carbon, and nitrogen and sulfur dioxides -- during the three days previous to patients'' hospital visits and then again at corresponding dates to determine whether these factors trigger severe headaches.

The findings showed that of all of the environmental factors considered, higher air temperature in the 24 hours prior to the patient's hospital visit was most closely associated with headache symptoms, with a 7.5 percent higher risk of severe headache reported for each temperature increase of 5 degrees Celsius (approximately 9 degrees Fahrenheit). To a lesser degree, lower barometric pressure 48 to 72 hours prior to patients'' emergency room visits also appeared to trigger headache.

The researchers found no evidence that air pollutants influenced the onset of headache, but could not rule out a smaller effect similar to that previously seen for stroke.